The device of the present invention relates to the continuous circulation of a large body of liquid material, and more particularly pertains to a liquid circulating device for continuously circulating and mixing sewage sludge thus facilitating the anaerobic digestion of the sludge to effect its safe treatment and disposal.
Over the last one hundred years, collection, treatment, processing, and disposal of human waste has improved immeasurably from the practice of dumping raw sewage in streams, rivers, lakes, and poorly managed, poorly located landfills. Modern sewage treatment facilities include a number of complex steps, procedures and stations for the treatment and disposal of human waste.
The anaerobic digestion of human waste, i.e., substantially liquid sludge, is necessary in the treatment process. In order for anaerobic digestion of the sewage sludge to occur, the sludge must be continuously or batch-fed into large sealed digester or holding tanks, varying in size from 25 to 125 feet in diameter with 15 to 50 foot sidewalls. The size and number of each digester tank is dependent on the city or municipality being served; a 250,000 gallon digester tank is a common size, although holding tanks can range in size from 50,000 gallons to 2,000,000 gallons.
Essential to the circulation, mixing, and anaerobic digestion of the sewage sludge is the placement of some type of open-ended conduit, draft tube, or stackpipe inside the digester tank and submerged within the body of primarily liquid material, such as sewage sludge. In addition, some type of bubble generator is attached to and is in flow communication with the stackpipe. A gas supply line feeding into the bubble generator causes the continual creation of gas bubbles inside the bubble generator. The gas bubbles are then introduced into the stackpipe for propelling the liquid material up through the stackpipe, thus effecting continuous circulation, mixing, and digestion.
The sludge is derived from raw sewage which has been allowed to settle or thicken in other parts of the facility, and then pumped into the digester tank as the feedstock, i.e., the predominantly liquid sewage sludge. The sewage sludge itself contains 92-98% liquid-type material and 2-8% solids and has a thick, soupy consistency. The continuous mixing and circulation of the sewage sludge by the liquid circulating device breaks the sludge down and enables various kinds of microbes to feed upon and digest the sludge. The sludge is actually digested by acid-forming microbes, and the waste material of the acid-forming microbes is eaten by methane-forming microbes, which produce methane as a by-product. One measure of the performance of a digester tank is the amount of methane gas derived from the digestion process: according to specific chemistry formulas used industry-wide, for so many pounds of waste, at a given set of conditions, a proportionate number of pounds of a given substance will be produced.
Concomitant with the continuous or batch feeding of the feedstock into the digester tank, thoroughly digested sludge is being pumped out of the digester tank for further treatment and eventual disposal. Depending on whether the treatment facility serves industrial users or rural users, the treated sludge is deposited in landfills or it can be recycled as fertilizer for farmland.
Representative of prior art liquid circulating devices are the three Lipert patents, U.S. Pat. No. 4,187,263, U.S. Pat. No. 4,293,506 and U.S. Pat. No. 4,356,131. U.S. Pat. No. 4,293,506 is a continuation-in-part of U.S. Pat. No. 4,187,263.
The Lipert U.S. Pat. No. 4,187,263, discloses a verticallyextending, open-ended stackpipe, a large bubble generator adjacent the stackpipe comprising a gas accumulator tank having an open bottom, a peripheral wall, and a top wall. A vertically-extending standpipe is positioned adjacent the bubble generator and the stackpipe, and allows the passage of liquid material therethrough. In addition, a T-pipe extends outwardly from the stackpipe and ends at a flared, downwardly-pointing frusto-conical opening. The open upper end of the standpipe is centered within the flared, frusto-conical opening of the T-pipe.
During operation of the Lipert device '263, gas is delivered into the gas accumulator tank by an inlet pipe. The gas pushes down the liquid sludge in the accumulator tank and also simultaneously lowers the sludge level in a bent pipe attached to, and in flow communication with, the standpipe and the gas accumulator tank. When the sludge reaches a certain predetermined level in the bent pipe and the gas accumulator tank, the gas is siphoned through the bent pipe into the standpipe, up through the transverse T-pipe and then upwardly through the stackpipe as a single large gas bubble. The upward movement of the gas bubble through the stackpipe pushes liquid sludge ahead of the bubble with a piston-like action upward and out the stackpipe upper end. The continual introduction of bubbles into the stackpipe causes the circulation of the liquid sludge through the stackpipe. Thus, the result is the continuous circulation, mixing, and digestion of the digester tank contents.
A number of factors and problems must be considered when designing and installing liquid circulating devices. The lengt of a stackpipe must be related to the volume and depth of the digester tank. A longer stackpipe provides better mixing because the bubble achieves greater momentum in its upward movement through the stackpipe. However, the longer the stackpipe, the more horsepower the compressor will require in order to generate the gas bubbles. Moreover, there is a physical relationship between the depth of the stackpipe and the compressor horsepower needed to generate the gas bubbles: the deeper the point at which the bubble enters the stackpipe, the more horsepower the compressor will require to generate that particular gas bubble.
In addition, the amount of bubbles cycling through the stackpipe at any one time depends on the length of the stackpipe, the depth the stackpipe is placed in the digester tank, and the bubble flow rate into the stackpipe. Each facility will have its own requirements based, in part, on the digester tank volume and the desired turnover rate of the feedstock.
Moreover, there is a trade-off between the gas pressure required to introduce the gas bubbles into the stackpipe and the rate of flow of the feedstock through the stackpipe. If the gas bubble generator is located high on the stackpipe, a lower pressure gas supply can be used, but an inadequate feedstock flow through the stackpipe will occur as well as the creation of malformed bubbles that may not fill the diameter of the stackpipe.
On the other hand, a gas bubble generator placed on the lower portion of the stackpipe will require gas supplied at a higher pressure and a compressor of greater horsepower, but a well-formed bubble will be generated as well as a greater flow rate and a more efficient mixing of the feedstock.
Also, the design of the liquid circulating device must consider the phenomenon known as ragging. Ragging is the term for pieces of fibrous material such as cloth, rags, hair, and fiber balls that clog and plug kitchen and bathroom drains. Ragging occurring in a liquid circulating device will internally clog parts of the device and obstruct the flow of liquid material therethrough, thus impeding the generation of properly formed gas bubbles into the stackpipe.
In the Lipert U.S. Pat. No. 4,187,263, there is a gap between the open upper end of the standpipe and the flared, frusto-conical opening of the transverse T-pipe that extends outwardly from the stackpipe. Ragging that occurs in this gap will impede the flow of gas bubbles or cause the gas bubbles to slip up the side of the stackpipe or down the side of the standpipe. Thus, the continuous circulation of the feedstock will be impeded and the performance of the liquid circulating device will be degraded.
These are some of the factors and problems that must be considered in the design and installation of liquid circulating devices for placement in a digester tank.